The present invention relates to workpiece positioning tables and, more particularly, to workpiece positioning tables requiring the highest accuracy and repeatability.
Workpiece positioning tables are employed to position an object along one, two or three axes for performing a function on the workpiece. For example, workpiece positioning tables are employed in the manufacture of integrated circuit chips. In such manufacture, a silicon chip to be treated is mounted on the table and the table is controlled in one, two, three or more dimensions to position specific portions of the workpiece with respect to a stationary reference frame. One such positioning table is disclosed in U.S. Pat. No. 4,013,280.
In the referenced U.S. patent, a table is guided along one axis by upwardly projecting arms of a base. Guidance is provided by facing V-notches in the table and in the upwardly projecting arms with crossed pairs of roller bearings bridging parallel opposed surfaces of the notches.
Although the crossed bearings of the referenced patent significantly improve guidance accuracy of positioning tables, a limit to the accuracy obtainable is enforced by the ability to accurately machine the V-notches and by the accuracy of roller bearings. The guidance inaccuracies arising from notch and roller bearing variations is exhibited in directions transverse to the motion of the table.
In addition to transverse inaccuracies, a positioning table can exhibit inaccuracies along the axis of motion. High precision positioning tables of the prior art have employed lead screws driven by a servomotor attached to the base driving a precision circulating-ball nut attached to the table. Such screw drive systems are affected by backlash and by shaft windup as well as by limited acceleration capability. In a prior U.S. Pat. Ser. No. 195,238 now U.S. Pat. No. 4,378,709, the inventor of the present application has disclosed a friction drive system which replaces the lead screw drive with direct friction drive from a motor shaft to a bar attached to the table. In a further U.S. Pat. Ser. No. 383,351, the inventor of the present application has disclosed a direct drive linear motor in which the driving coils are affixed to one of the base and the table and a set of permanent magnets is affixed to the other of the base and the table. With appropriate drive signals, extreme accuracy can be obtained without backlash.
The disclosures of the above U.S. patent and the two referenced U.S. patent applications are herein incorporated by reference and made a part hereof.
The improved drive systems disclosed in the above-referenced patent applications have made it possible to reduce longitudinal positioning errors to the point that transverse errors due to guidance inaccuracies now dominate. When the task to be done requires repeatable positioning to dimensions on the order of one micron or less (10.sup.-6 meters), such requirements lie beyond the leading edge of current accuracies in V-notch and roller bearing technology.
Manufacturing processes of solid-state devices are moving toward requirements for positioning on the micron and submicron order. As noted, present guidance techniques pollute the longitudinal drive accuracy to such an extent that the requirements of such solid-state processing cannot be met by the prior art.